Gay-marriage advocates have vowed to fight on after a bill allowing same-sex marriage was defeated.Source: AAP

BROKEN-HEARTED supporters of same-sex marriage have not given up their campaign, after federal MPs overwhelmingly defeated legislation to change the marriage act in the lower house.

Fifteen Gillard government ministers were among the 42 MPs who voted in favour of Labor backbencher Stephen Jones' bill, but coalition MPs not allowed a conscience vote swelled the numbers against the bill to 98.

The vote came as Liberal senator Cory Bernardi resigned from the coalition frontbench team, under pressure from Opposition Leader Tony Abbott, after he suggested allowing gay marriage could lead to polygamy and bestiality.

The Senate was on Wednesday still debating another private members' bill to legalise same-sex marriage, but it's also expected to be defeated this week.

Moves are under way in at least two states to endorse gay marriage, even though the marriage act is in the federal jurisdiction.

But Australian Marriage Equality national convenor, Alex Greenwich, said the 42-vote bloc in the lower house was a significant gain on the last bill, which received only six votes in favour.

"Now the federal parliament has effectively brushed the wishes of a majority of Australians aside, the states and territories will take the lead, making me confident we will see same-sex marriages performed somewhere in Australia by the end of the year," he said.

In NSW, a cross-party group of MPs is drafting a bill on the issue after Liberal Premier Barry O'Farrell and state opposition leader John Robertson granted a conscience vote to coalition and Labor members.

Tasmania's lower house passed a bill allowing gay marriage in August, and the upper house is due to consider it next week.

In federal parliament, the Australian Greens have vowed to leave their bill on the table until support for same-sex marriage builds.

Mr Jones urged gay marriage supporters to "maintain your rage".

"I'm quite confident in about 10 years' time some or all of us are going to be attending a wedding, a same-sex marriage, that will be both conducted and recognised here in Australia," he said.

"Change is inevitable."

Liberal frontbencher Malcolm Turnbull voted against the bill, but only because he would have had to resign his portfolio if he voted with his conscience.

He told reporters he would work with other MPs to bring on a bill to allow gay civil unions, as early as this year.

Leader of the House Anthony Albanese, who was among the Labor MPs to vote in favour of gay marriage, talked down the prospects of a civil union bill, saying gay and lesbian people wanted marriage equality and "not second best".

Greens deputy leader Adam Bandt said Opposition Leader Tony Abbott and Prime Minister Julia Gillard, who also voted against the bill, had "broken the hearts" of gay and lesbian couples and their families but they would not give up.

Mr Bandt, who failed in a last-minute bid to delay the vote until next year when the political climate might have been more favourable, said Mr Abbott's denial of a conscience vote for coalition MPs contributed to its failure.

"There are many in the Labor Party who just want to clear the decks before an election year and just push this off the agenda ... but equality is not going to go away", he said.

Ms Gillard and Mr Abbott had said during the 2010 federal election they would not support a change to marriage laws.

Australian Christian Lobby (ACL) managing director Jim Wallace said it was time to move on from the issue.

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